Memory Foam Mattress Not Expanding? What to Do, Hour by Hour

Quick answer: A memory foam mattress not expanding right away is normal. Full recovery usually takes 24 to 72 hours: Zinus says 48 to 72 hours, and Nectar recommends allowing 72. A cold room, a long time in the box or a poor base can slow it. Still well short of its listed height after about 3 days? Contact the manufacturer.

That first look can be a shock. You cut the plastic, the mattress unrolls, and it’s thinner than the one in the photos with a dent along one edge.

Give it time before you worry. Most of the rising happens on the first day, and the rest takes a couple more.

What Normal Expansion Looks Like

Here’s what manufacturers say to expect:

Time after unboxing	What the brands say
2 to 3 hours	Tuft & Needle suggests letting it air out and expand for 2 to 3 hours, though it’s ready to sleep on right away
A few hours	Zinus says most of its mattresses reach about 90% of their size
24 to 72 hours	Nectar’s full-expansion range
48 to 72 hours	Zinus’s figure for full recovery

When I compared the three brands’ figures, the ranges overlapped but didn’t match exactly. So treat your own brand’s number as the one that counts.

The good news: you don’t have to wait to use it. Nectar says sleeping on it right away won’t damage it or stop it from expanding. Zinus says it’s safe the first night, though it may feel a little firmer at first.

Why It Happens

If your memory foam mattress is not expanding on schedule, one of these is usually behind it:

1. Not enough time. The likeliest one, in my view. Day one isn’t the finish line.
2. A cold room. Zinus says a room around 70°F (21°C) speeds expansion, and cooler temperatures slow it down.
3. It sat in the box too long. Zinus says to open the box within 72 hours of delivery. Nectar says leaving a mattress compressed for an extended period can reduce its quality.
4. Humidity or poor airflow. Nectar lists air pressure, humidity and ventilation as factors.
5. An uneven base. Zinus says to lay the mattress flat on a supportive surface.
6. A genuine defect. Rare, but possible if one area never rises.

How to Fix a Memory Foam Mattress Not Expanding

Work through these in order.

1. Get It Out of the Box Fast

If it’s still boxed, open it today. Nectar goes further: its FAQ says mattresses not opened within 90 days of delivery are treated as misused and lose warranty and return eligibility. Other brands set their own limits, so check yours.

2. Lay It Flat on a Solid Base

Unroll it onto the foundation or frame you’ll actually use. Zinus recommends a supportive, even surface. Avoid leaving it propped against a wall or draped over a too-small frame. My reasoning: foam settles into whatever shape it’s resting on.

3. Warm the Room

This is the quickest win in winter. Bring the room up to about 70°F (21°C), as Zinus suggests, and keep it there for the first day or two. In practice, I’d be most suspicious of a mattress left in a cold garage or an unheated spare room.

4. Let It Breathe

Take off all the plastic, and leave the bedding off for the first few hours. Tuft & Needle recommends a 2 to 3 hour air-out. Open a window or run a fan if the room is stuffy, since Nectar mentions ventilation as a factor.

5. Work the Stubborn Spots Gently

Zinus suggests gently rolling or walking across the surface to help it expand, and using a handheld steamer on stubborn corners. Be gentle. My take: if your brand’s instructions don’t mention steam, I’d skip it and stick to time and warmth.

You can also just sleep on it. Both Nectar and Zinus say first-night use is fine.

6. Measure After 72 Hours

Once 3 days have passed, measure the height at the center and at each corner. Compare it with the height listed for your model.

Don’t panic over a slight difference. Look for an area that’s clearly lower than the rest, or a mattress that’s well short of its spec everywhere.

When to Contact the Manufacturer

Get in touch if, after about 72 hours in a warm room:

• one section or corner is still sunken
• the whole mattress is clearly below its listed height
• you see a crease or fold line that isn’t easing

Nectar says that if a mattress never reaches full height, you should contact them right away. Zinus offers its sleep specialists by chat or email. Take photos with a tape measure in frame, note the delivery date, and keep the receipt.

Contact them well inside your trial window. That gives you the most options.

If your mattress is older and has developed a dip rather than failing to rise, that’s a different problem; see how to fix memory foam mattress sags. And if you’re checking the size, our guide to what size mattress is 60x80 has standard dimensions. More help lives in our mattresses and toppers section.

Frequently Asked Questions

How long does it take a memory foam mattress to expand?

Usually 24 to 72 hours. Zinus says 48 to 72 hours for full recovery, with about 90% in a few hours. Nectar recommends giving it 72 hours.

Can I sleep on my mattress before it fully expands?

Yes. Nectar says it won’t damage the mattress or stop it expanding, and Zinus says it’s safe the first night, though it may feel firmer.

Why is my memory foam mattress not expanding in the corners?

Zinus singles out corners as the stubborn spots. Give it the full 72 hours in a warm room. Zinus suggests gently working the surface and using a handheld steamer on stubborn corners.

Does a cold room stop a mattress from expanding?

It slows it down. Zinus says a room around 70°F (21°C) speeds expansion, while cooler temperatures slow it.

What if my mattress never fully expands?

Contact the manufacturer. Nectar says to reach out right away if a mattress never reaches full height. Bring photos, measurements and your receipt.

Last updated: November 10, 2026. Sources: Zinus: how long a mattress takes to expand, Nectar: how long a mattress takes to expand and Tuft & Needle: unboxing instructions.